Old thread, I realize, but in case anyone is still interested... There's really not that much variation in how you'd design a pre-amp stage with a single 12AX7. If anybody is interested, comment back and I'll post a schematic. I can't say exactly what values the original SVPre switched in and out with his different color coded configuration switches, but I can guess what circuit elements he was playing with. You could build one and swap parts til you get the sound you like, or put switches in with a range of possible values.